1. Field of the Invention
The present invention relates to water-soluble acid-type carboxymethyl cellulose which is useful as dispersant, carrier, coating agent and various other materials and also as an intermediate for carboxymethyl cellulose derivatives, and to a process for preparing the same.
2. Description of the Prior Art
Carboxymethyl cellulose (hereinafter referred to as "CMC") is a cellulose ether prepared by reacting cellulose with monochloroacetic acid in the presence of an alkali and is generally in wide use as a water-soluble polymeric electrolyte, for example, as thickner, dispersant, protective colloid and adhesive. CMC is commercially available usually in the form of a sodium salt, while an ammonium salt and a calcium salt thereof are also available as commercial products for some uses. The calcium salt, which is insoluble in water, is used chiefly as a disintegrator.
Thus, CMC is commercially available usually in the form of a salt-type CMC, whereas CMC of the acid type which is prepared by converting some or all of the salt-type carboxymethyl groups to groups of the free acid type is not provided as a commercial product.
This is due to the reason that the acid-type CMC obtained by the conventional process (wherein salt-type CMC which is 0.5 to 1.7 in the degree of substitution by carboxymethyl groups is modified with an acid as disclosed, for example, in Kagaku Kogyo Shiryo (Chemical Industry Data), 30, 191-204, (1962)), whether of the total acid type or of the partial acid type, is soluble neither in water nor in organic solvents, is therefore difficult to handle and finds limited use only as an absorbent material (e.g. Published Unexamined Japanese Patent Application SHO 56-28755), etc.
On the other hand, it is likely that a very small amount of acid-type carboxymethyl groups is detected in commercial alkali salt-type CMC's. When the amount is at least 2% of the total degree of substitution by carboxymethyl groups, the CMC is partly insoluble in water, while if the amount is at least 5%, the CMC is almost insoluble in water. This appears attributable to the insolubilization of CMC that could result, for example, when some of the acid-type carboxymethyl groups underogo ester linkage (crosslinking) with hydroxyl groups in the same molecule and adjacent molecules during the conventional process for preparing the CMC, especially during the drying step.
Thus the conventional acid-type CMC is thought to be insoluble in water and also in organic solvents.